Front Contact Solar Cell With Formed Electrically Conducting Layers On the Front Side And Backside

ABSTRACT

A bipolar solar cell includes a backside junction formed by a silicon substrate and a first doped layer of a first dopant type on the backside of the solar cell. A second doped layer of a second dopant type makes an electrical connection to the substrate from the front side of the solar cell. A first metal contact of a first electrical polarity electrically connects to the first doped layer on the backside of the solar cell, and a second metal contact of a second electrical polarity electrically connects to the second doped layer on the front side of the solar cell. An external electrical circuit may be electrically connected to the first and second metal contacts to be powered by the solar cell.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to solar cells, and moreparticularly but not exclusively to solar cell fabrication processes andstructures.

2. Description of the Background Art

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor wafer usingsemiconductor processing technology. A solar cell includes P-type andN-type diffusion regions that form a junction. Solar radiation impingingon the solar cell creates electrons and holes that migrate to thediffusion regions, thereby creating voltage differentials between thediffusion regions. In a backside contact solar cell, both the diffusionregions and the metal contacts coupled to them are all on the backsideof the solar cell. The metal contacts allow an external electricalcircuit to be coupled to and be powered by the solar cell.

In a front contact solar cell, at least one of the metal contacts makingan electrical connection to a diffusion region is on the front side ofthe solar cell. While backside contact solar cells have an aestheticadvantage over front contact solar cells due to the absence of metalcontacts on the front side, and are thus preferred for residentialapplications, aesthetics is not a major requirement for power plants andother applications where power generation is the main concern. Disclosedherein are structures for a relatively efficient and cost-effectivefront contact solar cell and processes for manufacturing same.

SUMMARY

In one embodiment, a bipolar solar cell includes a backside junctionformed by a silicon substrate and a first doped layer of a first dopanttype on the backside of the solar cell. A second doped layer of a seconddopant type makes an electrical connection to the substrate from thefront side of the solar cell. A first metal contact of a firstelectrical polarity electrically connects to the first doped layer onthe backside of the solar cell, and a second metal contact of a secondelectrical polarity electrically connects to the second doped layer onthe front side of the solar cell. For example, the first doped layer maybe polysilicon doped with a P-type dopant, while the second doped layermay be polysilicon doped with an N-type dopant. An external electricalcircuit may be electrically connected to the first and second metalcontacts to be powered by the solar cell.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a cross-section of a solar cell in accordancewith an embodiment of the present invention.

FIG. 2 is a plan view schematically showing the front side of the solarcell of FIG. 1 in accordance with an embodiment of the presentinvention.

FIG. 3 is a plan view schematically showing the backside of the solarcell of FIG. 1 in accordance with an embodiment of the presentinvention.

FIGS. 4-19 schematically illustrate the fabrication of the solar cell ofFIG. 1 in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of apparatus, process parameters, materials, process steps,and structures, to provide a thorough understanding of embodiments ofthe invention. Persons of ordinary skill in the art will recognize,however, that the invention can be practiced without one or more of thespecific details. In other instances, well-known details are not shownor described to avoid obscuring aspects of the invention.

The present invention pertains to a low cost, high-efficiency frontcontact solar cell. Another such front contact solar cell is alsodisclosed in commonly-owned U.S. application Ser. No. 12/070,742,entitled “FRONT CONTACT SOLAR CELL WITH FORMED EMITTER,” filed by PeterJohn Cousins on Feb. 20, 2008, having Express Mail No. EM 142856406US.

FIG. 1 schematically shows a cross-section of a bipolar front contactsolar cell 100 in accordance with an embodiment of the presentinvention. The solar cell 100 has a front side where metal contacts102are located and a backside on a same side as the metal contact 110. Thefront side, which is opposite the backside, faces the sun during normaloperation to collect solar radiation. The front side of the solar cellincludes layers formed on the front side surface of a substrate 101, andthe backside of the solar cell includes layers formed on the backsidesurface of the substrate 101.

In the example of FIG. 1, the solar cell 100 includes a backsidejunction formed by a P-type doped polysilicon layer 108 serving as aP-type diffusion region and the N-type silicon substrate 101 servings asan N-type diffusion region. In one embodiment, the polysilicon layer 108has an emitter saturation current of about 10 fA·cm⁻² The N-type siliconsubstrate 101 may comprise a long lifetime (e.g., 2 to 5 ms) N-typesilicon wafer and may have a thickness of about 100 to 250 μm asmeasured from the backside surface to a tip of the textured front sidesurface of the substrate (see dimension D24). The front side surface ofthe substrate 101 may be randomly textured (labeled as 113) and includesan N-type doped region 105 formed in the substrate. The N-type dopedregion 105 provides low front surface recombination and improves lateralconductivity whilst not compromising the blue response of the solarcell. The N-type doped region 105 may have a sheet resistance of 100 to500Ω/sq.

An N-type doped polysilicon layer 106 on the front side of the solarcell 100 provides low contact resistance and minimizes contactrecombination. The polysilicon layer 106 is also referred to as an“N-dot” because, in one embodiment, it forms a dot-shape to minimize thearea of heavily diffused regions on the front surface of the substrate101. In one embodiment, the polysilicon layer 106 has an emittersaturation current of about 5 fa·cm⁻². The polysilicon layer 106 may beformed on a thin dielectric layer 402. The thin dielectric layer 402 maycomprise silicon dioxide thermally grown to a thickness of about 10 to50 Angstroms on the front side surface of the substrate 101. In oneembodiment, the thin dielectric layer 402 is not on the textured frontside surface 113 because it is removed by the texturing step.

An antireflective coating (ARC) of silicon nitride layer 103 may beformed on the textured front side surface 113 of the substrate 101. Thetexture front side surface 113 and the antireflective coating helpimprove solar radiation collection efficiency. A passivating oxide 135may be formed on the textured front side surface 113 under the siliconnitride layer 103. In one embodiment, the passivating oxide 135 isthermally grown on the textured front side surface of the substrate 101to a thickness of about 10 to 250 Angstroms.

In one embodiment, the polysilicon layer 108 is formed on a thindielectric layer 107. The polysilicon layer 108 may be formed by forminga layer of polysilicon by Chemical Vapor Deposition (CVD), such as LowPressure CVD (LPCVD) or Plasma Enhanced CVD (PECVD), and thermal anneal.The polysilicon layer 108 may have a sheet resistance of about 100Ω/sq.The thin dielectric layer 107 may comprise silicon dioxide thermallygrown to a thickness of about 10 to 50 Angstroms on the backside surfaceof the substrate 101. A metal contact 110 electrically connects to thepolysilicon layer 108 through contact holes 123 formed through adielectric comprising a silicon dioxide layer 109. The silicon dioxidelayer 109 provides electrical isolation and allows the metal contact 110to serve as an infrared reflecting layer for increased solar radiationcollection. As will be more apparent below, the silicon dioxide layer109 may also serve as a dopant source in the diffusion of dopants intothe polysilicon layer 108.

In one embodiment, the metal contact 110 comprises aluminum having asheet resistance of about 15 mΩ/sq and formed to a thickness of about 10to 30 μm by a printing process. A bus bar 112 electrically connects tothe metal contact 110 to provide a positive polarity terminal forelectrically connecting an external electrical circuit to the solar cell100. In one embodiment, the bus bar 112 comprises silver and has aconductance of about 5-25 mΩ·cm and a thickness of about 15-35 μm.

The metal contact 110 may also comprise a stack of materials comprisingaluminum formed towards the substrate 101, a diffusion barrier layercomprising titanium-tungsten formed on the aluminum, and a seed layer ofcopper formed on the diffusion barrier layer. In that configuration, thebus bar 112 may comprise copper electroplated onto the seed layer.

On the front side of the solar cell 100, each metal contact 102electrically connects to the polysilicon layer 106 through a contacthole 120 formed through the silicon dioxide layer 104. Similar to thesilicon dioxide layer 109, the silicon dioxide layer 104 may serve as adopant source in diffusing dopants to the polysilicon layer 106. As canbe appreciated, the polysilicon layers 108 and 106 may also be pre-dopedbefore formation on the substrate 101.

A metal contact 102 provides a negative polarity terminal to allow anexternal electrical circuit to be coupled to and be powered by the solarcell 100. In one embodiment, the metal contact 102 comprises silverhaving a sheet resistance of about 5-25 mΩ·cm and a thickness of about15-35 μm.

The pitch between adjacent metal contacts 102 separated by a texturedfront side surface 113 (see dimension D21) may be about 4200 μm. Thediameter of a polysilicon layer 104 (see dimension D22) may be about 390μm.

FIG. 2 is a plan view schematically showing the front side of the solarcell 100 in accordance with an embodiment of the present invention. Inthe example of FIG. 2, two bus bars 201 run parallel on the front sideof the substrate 101. The contact holes 120, in which the metal contacts102 are formed, may each have a diameter of about 50 to 200 μm. Aplurality of metal contacts 102 is formed perpendicular to the bus bars201. Each metal contact 102 may have a width of about 60-120 μm (seedimension D23 in FIG. 1).

FIG. 3 is a plan view schematically showing the backside of the solarcell 100 in accordance with an embodiment of the present invention. Inthe example of FIG. 3, the bus bars 112 run parallel on the backside. Inpractice, the bus bars 201 and 112 will be electrically connected tocorresponding bus bars of adjacent solar cells to form an array of solarcells.

Solar cells have gained wide acceptance among energy consumers as aviable renewable energy source. Still, to be competitive with otherenergy sources, a solar cell manufacturer must be able to fabricate anefficient solar cell at relatively low cost. With this goal in mind, aprocess for manufacturing the solar cell 100 is now discussed withreference to FIGS. 4-19.

FIGS. 4-19 schematically illustrate the fabrication of the solar cell100 in accordance with an embodiment of the present invention.

In FIG. 4, an N-type silicon substrate 101 is prepared for processinginto a solar cell by undergoing a damage etch step. The substrate 101 isin wafer form in this example, and is thus typically received withdamaged surfaces due to the sawing process used by the wafer vendor toslice the substrate 101 from its ingot. The substrate 101 may be about100 to 250 microns thick as received from the wafer vendor. In oneembodiment, the damage etch step involves removal of about 10 to 20 μmfrom each side of the substrate 101 using a wet etch process comprisingpotassium hydroxide. The damage etch step may also include cleaning ofthe substrate 101 to remove metal contamination.

In FIG. 5, thin dielectric layers 402 and 107 are formed on the frontand back surfaces, respectively, of the substrate 101. The thindielectric layers 402 and 107 may comprise silicon dioxide thermallygrown to a thickness of about 10 to 50 Angstroms on the surfaces of theN-type silicon substrate 101. A layer of polysilicon is then formed onthe thin dielectric layers 402 and 107 to form the polysilicon layer 106on the front side and the polysilicon layer 108 on the backside,respectively. Each of the polysilicon layer 106 and the polysiliconlayer 108 may be formed to a thickness of about 1000 to 2000 Angstromsby CVD.

In FIG. 6, a P-type dopant source 403 is formed on the polysilicon layer108 on the backside of the solar cell. As its name implies, the P-typedopant source 403 provides a source of P-type dopants for diffusion intothe polysilicon layer 108 in a subsequent dopant drive-in step. Adielectric capping layer 404 is formed on the P-type dopant source 403to prevent dopants from escaping from the backside of the solar cellduring the drive-in step. In one embodiment, the P-type dopant source403 comprises BSG (borosilicate glass) deposited to a thickness of about500 to 1000 Angstroms by atmospheric pressure CVD (APCVD) and has adopant concentration of 5 to 1% by weight, while the capping layer 404comprises undoped silicon dioxide formed to a thickness of about 2000 to3000 Angstroms also by APCVD. Formation of the P-type dopant source 403and the capping layer 404 may be followed by a cleaning step to removepossible contamination from the front side of the source cell inpreparation for formation of an N-type dopant source on the front sideof the solar cell.

In FIG. 7, an N-type dopant source 405 is formed on the polysiliconlayer 106 on the front side of the solar cell. As its name implies, theN-type dopant source 405 provides a source of N-type dopants fordiffusion into the polysilicon layer 106 in a subsequent dopant drive-instep. A dielectric capping layer 406 is formed on the N-type dopantsource 405 to prevent dopants from escaping from the front side of thesolar cell during the drive-in step. In one embodiment, the N-typedopant source 405 comprises PSG (phosphosilicate glass) deposited to athickness of about 500 to 1000 Angstroms by atmospheric pressure CVD(APCVD) and has a dopant concentration of 5 to 10% by weight, while thecapping layer 406 comprises undoped silicon dioxide formed to athickness of about 2000 to 3000 Angstroms also by APCVD.

In FIG. 8, a mask 407 is formed on the capping layer 406 on the frontside of the solar cell. The mask 407 defines and protects regions wheremetal contacts 102 will be formed during subsequent etching of thecapping layer 406 and N-type dopant source 405.

In FIG. 9, a mask 408 is formed on the capping layer 404 on the backsideof the solar cell. The mask 408 protects the backside surface of thesolar cell during etching of the capping layer 406 and the N-type dopantsource 405 on the front side of the solar cell. In the example of FIG.9, edges of the backside of the solar cell are not covered, i.e.,exposed, by the mask 408 to allow for formation of an isolation trenchon the edges of the solar cell. The masks 407 and 408 may comprise anacid resistance organic material, such as a resist, and formed using aprinting process, such as screen printing or inkjet printing.

In FIG. 10, portions of the capping layer 406 and N-type dopant source405 not covered by the mask 407 (see FIG. 9) and portions of the cappinglayer 404 and P-type dopant source 403 not covered by the mask 408 areetched in an oxide etch step. The oxide etch step exposes the regionwhere the textured front side surface 113 (see FIG. 1) is subsequentlyformed. In one embodiment, the oxide etch step comprises a BOE (bufferedoxide etch) process. The masks 407 and 408 are removed after the oxideetch step.

In FIG. 11, exposed portions of the substrate 101 on the front side arerandomly textured to form the textured front side surface 113. In oneembodiment, the front side surface of the substrate 101 is textured withrandom pyramids using a wet etch process comprising potassium hydroxideand isopropyl alcohol. The texturing process etches away exposedportions of the polysilicon layer 106.

In FIG. 12, a dopant drive-in step is performed to diffuse N-typedopants from the N-type dopant source 405 (see FIG. 11) into thepolysilicon layer 106, to diffuse P-type dopants from the P-type dopantsource 403 to the polysilicon layer 108, and to diffuse N-type dopantsinto the front side of the substrate 101 to form the N-type doped region105. Silicon dioxide layer 109 represents the P-type dopant source 403and the capping layer 404 after the drive-in step. Similarly, silicondioxide layer 104 represents the N-type dopant source 405 and thecapping layer 406 after the drive-in step.

The polysilicon layer 108 becomes a P-type doped layer and thepolysilicon layer 106 becomes an N-type doped layer after the drive-instep. The N-type doped region 105 may be formed by exposing the sampleof FIG. 11 to phosphorus in the diffusion furnace during the drive-instep, for example. The use of the N-type dopant source 405, instead ofsimply exposing the polysilicon layer 106 in a phosphorus environment,advantageously allows for a more controlled and concentrated N-typediffusion to the N-type doped polysilicon layer 106. A passivating oxidelayer (not shown in FIG. 11; see layer 135 in FIG. 1) may be grown onthe textured surface 113 during the drive-in process.

In one embodiment, the drive-in step to dope the polysilicon layer 108on the backside, to dope the polysilicon layer 106 on the front side,and to form the N-type doped region 105 may be formed in-situ, which inthe context of the present disclosure means a single manual (i.e., byfabrication personnel) loading of the substrate 101 in a furnace orother single chamber or multi-chamber processing tool. In oneembodiment, the drive-in step is performed in a diffusion furnace. Thepreceding sequence of steps leading to the drive-in step allows forin-situ diffusion, which advantageously helps in lowering fabricationcost.

In FIG. 13, the antireflective coating of silicon nitride layer 103 isformed over the textured front side surface 113. The silicon nitridelayer 103 may be formed to a thickness of about 450 Angstroms by PECVD,for example.

In FIG. 14, a mask 409 is formed on the front side of the solar cell.The mask 409 defines the regions where the contact holes 120 (seeFIG. 1) will be subsequently formed.

In FIG. 15, a mask 410 is formed on the backside of the solar cell. Themask 410 defines the regions where the contact holes 123 (see FIG. 1)will be subsequently formed. The masks 409 and 410 may comprise an acidresistance organic material, such as a resist, and formed using aprinting process, such as screen printing or inkjet printing.

In FIG. 16, contact holes 120 and 123 are formed by removing portions ofthe silicon dioxide layers 104 and 109 exposed through the masks 409 and410, respectively. In one embodiment, the contact holes 120 are formedby using a selective contact etch process that removes exposed portionsof the silicon dioxide layer 104 and stops on the polysilicon layer 106.The same contact etch process removes exposed portions of the silicondioxide layer 109 and stops on the polysilicon layer 108. In oneembodiment, the contact etch process comprises a BOE (buffered oxideetch) process. The masks 409 and 410 are removed after the contact etchprocess.

In FIG. 17, the metal contact 110 is formed on the silicon dioxide layer109 to fill the contact holes 123 and make electrical connection to thepolysilicon layer 108. The metal contact 110 may be formed using aprinting process, such as screen printing. The metal contact 110 maycomprise aluminum, which, together with the silicon dioxide layer 109,makes an excellent backside infrared reflector to increase solarradiation collection efficiency.

In FIG. 18, the metal contact 123 is formed on the metal contact 110 tomake electrical connection to the polysilicon layer 108. The metalcontact 123 may be formed using a printing process, such as screenprinting. The metal contact 110 may comprise silver, for example.

In FIG. 19, the metal contacts 102 are formed on the silicon dioxidelayer 104 to fill the contact holes 120 and make electrical connectionto the substrate 101 by way of the polysilicon layer 106. The metalcontacts 120 may comprise silver and formed using a printing process,such as screen printing.

Formation of the metal contacts 102 and 110 may be followed by a firingstep. The firing step is applicable when using screen printed silverpaste as metal contacts, but not when using other processes or metals.The solar cell 100 may then be visually inspected and tested.

An improved front contact solar cell and method of manufacturing samehave been disclosed. While specific embodiments of the present inventionhave been provided, it is to be understood that these embodiments arefor illustration purposes and not limiting. Many additional embodimentswill be apparent to persons of ordinary skill in the art reading thisdisclosure.

1. A solar cell having a front side facing the sun to collect solarradiation during normal operation and a backside opposite the frontside, the solar cell comprising: a silicon substrate; a first layer ofdoped polysilicon formed over a back surface of the substrate, the firstlayer of doped polysilicon forming a backside junction with thesubstrate; a second layer of doped polysilicon formed over a frontsurface of the substrate, the second layer of doped polysilicon makingan electrical connection to the substrate; a first dielectric layerbetween the first layer of doped polysilicon and the back surface of thesubstrate; a second dielectric layer between the second layer of dopedpolysilicon and the front surface of the substrate; a first metalcontact making an electrical connection to the first layer of dopedpolysilicon on the backside of the solar cell; and a second metalcontact making an electrical connection to the second layer of dopedpolysilicon on the front side of the solar cell, the first metal contactand the second metal contact being configured to allow an externalelectrical circuit to be powered by the solar cell.
 2. The solar cell ofclaim 1 further comprising an antireflective layer over a textured frontsurface of the substrate.
 3. The solar cell of claim 2 wherein theantireflective layer comprises silicon nitride.
 4. The solar cell ofclaim 1 wherein the substrate comprises an N-type silicon substrate, thefirst layer of doped polysilicon comprises a P-type doped polysilicon,and the second layer of doped polysilicon comprises an N-type dopedpolysilicon.
 5. The solar cell of claim 1 wherein the first metalcontact comprises aluminum formed over the first dielectric layer, thefirst dielectric layer comprising silicon dioxide.
 6. The solar cell ofclaim 1 wherein the first dielectric layer comprises silicon dioxideformed to a thickness between 10 and 50 Angstroms.
 7. The solar cell ofclaim 1 further comprising a third metal contact formed over the firstmetal contact.
 8. The solar cell of claim 1 further comprising an oxidelayer formed over the first layer of doped polysilicon and wherein thefirst metal contact forms an infrared reflecting layer with the oxidelayer on the backside of the solar cell.
 9. A method of fabricating asolar cell having a front side facing the sun to collect solar radiationduring normal operation and a backside opposite the front side, themethod comprising: forming a first layer of material over a back surfaceof an N-type silicon substrate on the backside of the solar cell;forming a first dopant source layer comprising P-type dopants over thefirst layer of material; forming a second layer of material over a frontsurface of the N-type silicon substrate on the front side of the solarcell; forming a second dopant source layer comprising N-type dopantsover the second layer of material; diffusing P-type dopants from thefirst dopant source layer to the first layer of material to form abackside junction with the silicon substrate; and diffusing N-typedopants from the second dopant source layer to the second layer ofmaterial.
 10. The method of claim 9 further comprising: forming a firstcapping layer over the first dopant source layer and a second cappinglayer over the second dopant source layer prior to diffusing dopantsfrom the first dopant source layer and the second dopant source layer.11. The method of claim 9 wherein the first dopant source layercomprises borosilicate glass.
 12. The method of claim 9 wherein thesecond dopant source layer comprises phosphosilicate glass.
 13. Themethod of claim 9 wherein the first and second layers of materialcomprise polysilicon.
 14. The method of claim 9 further comprising:texturing the front surface of the N-type silicon substrate; and formingan antireflective layer over the textured front surface of the N-typesilicon substrate.
 15. The method of claim 14 wherein the antireflectivelayer comprises silicon nitride.
 16. The method of claim 9 wherein thediffusion of the P-type dopants from the first dopant source layer tothe first layer of material and the diffusion of the N-type dopants fromthe second dopant source layer to the second layer of material areperformed in situ.
 17. A solar cell having a front side facing the sunto collect solar radiation during normal operation and a backsideopposite the front side, the solar cell comprising: an N-type siliconsubstrate; a textured surface on the N-type silicon substrate on thefront side of the solar cell; an antireflective layer over the texturedsurface of the N-type silicon substrate; a P-type polysilicon layerforming a backside junction with the N-type silicon substrate; an N-typepolysilicon layer over the front side of the solar cell; a negativepolarity metal contact making an electrical connection to the N-typepolysilicon layer from the front side of the solar cell; and a positivepolarity metal contact making an electrical connection to the P-typepolysilicon layer from the backside of the solar cell.
 18. The solarcell of claim 17 wherein the antireflective layer comprises a layer ofsilicon nitride.
 19. The solar cell of claim 17 further comprising afirst dielectric capping layer over the P-type polysilicon layer and asecond dielectric capping layer over the N-type polysilicon layer. 20.The solar cell of claim 17 wherein the positive polarity metal contactcomprises a metal forming an infrared reflecting layer with a dielectriclayer that is formed over the P-type polysilicon layer.